THAT MEAN OLD MAN’S HOUSE: Cheerful schoolgirls yesterday stroll past the high-walled compound in Abbottabad, Pakistan, where Osama bin Laden was killed by United States special forces in the early hours on Monday. Photo: AFP/Getty Images

Slain al Qaeda founder Osama bin Laden was plotting to derail American trains on the 10th anniversary of 9/11, documents seized from his Pakistani hideout reveal, officials said yesterday.

The plot — which indicates bin Laden was intimately involved with planning attacks right up until his death — was to sabotage the rails, forcing trains off the tracks on a bridge or over a chasm, the documents show.

A senior homeland-security official told The Post this morning that there is no plan to raise the terror-alert level because, he insisted, the rail plot was far too vague to translate into action because no actual plan had been formulated. The official insisted that the increased presence of police and other security teams at train stations throughout the region was little more than public relations designed to keep jittery commuters calm and to help justify ever-growing budgets for police and homeland-security agencies, which are facing steep funding cuts.

The official stressed, however, that the vulnerability of the nation’s rail system is real and he hoped that the latest news out of Pakistan will force the feds to insist on better security on rail lines and at train depots — commuter and cargo — around the U.S. Citing statistics that show $1 is spent on rail security for every $9 spent on aircraft security, he said “maybe they really will do something now. Rails are much more vulnerable and affect many more people. Most people in this country don’t travel by air and, the ones that do, fly maybe three times a year. But trains are everywhere, in every community and it’s every day.”

Since 9/11, security measures have been increased along rail lines across the country. More fences have been built, hidden security cameras have been added and, for some dangerous cargo loads, trains actually have minute-by-minute escorts armed to the teeth. But, he said, the rail system is still vulnerable because there are too many miles of track going through too many places to be fully secured. An attack on the rail lines also would likely prove more frightening because it would affect everyone in the country, not just those who live in big cities or near major airports.

Although considered in the “aspirational” phase, the information was disturbing enough that the FBI and Department of Homeland Security issued a nationwide alert.

“As of February 2010, al-Qa’ida was allegedly contemplating conducting an operation against trains at an unspecified location in the United States on the 10th anniversary of September 11, 2001,” the warning states.

“As one option, al-Qa’ida was looking into trying to tip a train by tampering with the rails so that the train would fall off the track at either a valley or a bridge.”

DHS said there was no “actionable intelligence” in the documents.

“The rail system and the train system we know is a top target. But this is the first time we’ve seen al Qaeda specifying trains and rails. There are vulnerabilities there,” a federal counterterrorism official told The Post.

Another official added that the intelligence confirmed “chatter” intercepted in February 2010.

Navy SEAL Team Six seized 100 thumb drives, DVDs and computer disks, along with 10 computer hard drives and five computers from Osama’s Abbottabad compound.

Included in the information gathered at the compound was a terrorist wish list — though it hasn’t revealed any specific plan so far, a US official said.

The official said documents indicated a desire to hit the United States with large-scale attacks in major cities and on key dates such as anniversaries and holidays. But there was no sign those plans were anything more than ambitions.

Even before the raid, intelligence officials for years have warned that al Qaeda is interested in attacking major cities on holidays, anniversaries and other dates that are uniquely American.

“We have no information of any imminent terrorist threat to the US rail sector, but wanted to make our partners aware of the alleged plotting; it is unclear if any further planning has been conducted,” said DHS spokesman Matthew Chandler, adding there is no threat level change.

Terrorists have exploited vulnerabilities in the train systems in the past, including the al-Qaeda-inspired 2004 Madrid rail bombings, which killed 191, and the 2005 London transit attacks, in which 56 perished.

Meanwhile, new details about the raid emerged yesterday, with Sen. Saxby Chambliss (R-Ga.) saying the SEAL assault team missed its first shot at bin Laden, but nailed him on the second, the Atlanta Journal-Constitution said.